Currently, in the architecture of LANs (local area networks) on computer networks, communication systems with better flexibility and higher mobility have been developed using, for example, wireless LANs with micro waves. Furthermore, a short-range wireless technology called Bluetooth, which is said to represent a WPAN (wireless personal area network), is used instead of wired cables. In an environment in which a large number of such technologies are used together with one another, there are problems in that electromagnetic waves interfere with each other when electromagnetic waves in the same band are used or in that transmission errors (multi-path) are caused by reflected waves and the like. More specifically, the transmission speed between equipment using the wireless technologies is lowered, and BER (bit error rate) is deteriorated, and in the worst case, the equipment may malfunction.
In order to solve these problems, conventionally, magnetic loss materials represented by ferrite and dielectric loss materials represented by carbon black, for example, have been used as electromagnetic wave absorbing materials. In order to supply these electromagnetic wave absorbing materials, electromagnetic wave absorbers having a predetermined complex relative dielectric constant and complex relative permeability have been developed. In this conventional technique, for example, when an electromagnetic wave absorber is realized using ferrite as a constituent in the 2.45 GHz band of a wireless LAN and the like, there is a problem in that due to the restriction of the snoek peak, it is usually impossible to reduce the thickness of the electromagnetic wave absorbing material to 4 mm or less.
As other conventional techniques, multi-layered electromagnetic wave absorbers including patterned layers have been disclosed in Japanese Unexamined Patent Publication JP-A 06-164184 (1994), Japanese Patent No. 3076473 (Japanese Unexamined Patent Publication JP-A 06-244583 (1994)), Japanese Patent No. 3209456 (Japanese Unexamined Patent Publication JP-A 06-140787 (1994)), Japanese Patent No. 3209453 (Japanese Unexamined Patent Publication JP-A 06-45782 (1994)), Japanese Unexamined Patent Publication JP-A 06-252582 (1994), Japanese Unexamined Patent Publication JP-A 06-224586 (1994), and Japanese Unexamined Patent Publication JP-A 09-148782 (1997). An electromagnetic wave absorber having a patterned layer based on a conductor loop structure has been disclosed in Japanese Unexamined Patent Publication JP-A 10-224075 (1998). Furthermore, a plurality of resonant frequency selective electromagnetic wave blocking planar members are described in Japanese Unexamined Patent Publication JP-A 11-204984 (1999) and Japanese Unexamined Patent Publication JP-A 11-195890 (1999). Moreover, a method for designing the shape of patterns and the like is described in Japanese Unexamined Patent Publication JP-A 2003-243876.
In JP-A 06-164184, a radio wave reflection preventing member using a patterned metal layer is described. In order to leave a space between molded sheet layers, this radio wave reflection preventing member has a configuration in which paper, cloths, non-woven fabrics or porous sheets are impregnated with paint containing a material such as ferrite and carbon. Thus, this technique is different from the invention in which these impregnated members are not used. Similar configurations are described as patterned resin layers also in JP-A 06-252582 and JP-A 06-224568, but these configurations are also different from that of at least one embodiment of the invention.
In Japanese Patent No. 3076473 (JP-A 06-244583), a radio wave absorber is described in which a set of a patterned layer and a resin layer is taken as one unit and this unit is repeated for a plurality of times. It is described that in the radio wave absorber, at least one coating film is used as the patterned layer, and thus this technique is different from at least one embodiment of the invention in which an element made of metal is used and there is a limitation regarding the conductivity.
In Japanese Patent No. 3209456 (JP-A 06-140787), a laminated layer member made of a patterned layer and a resin layer is described. This laminated layer member has a configuration in which patterned structures are piled up, and is different from at least one embodiment of the invention in the shape of an element. A similar configuration is described also in Japanese Patent No. 3209453 (JP-A 06-45782), but this form is also different from at least one embodiment of the invention in the shape of an element.
In JP-A 09-148782, JP-A 10-224075, JP-A 11-204984, and JP-A 11-195890, configurations are described in which a patterned layer and an insulation spacer material that may be a space are laminated. In these laminated configurations, a loss component based on thermal conversion such as a dielectric loss material or a magnetic loss material is not used, and thus this technique is different from at least one embodiment of the invention. Since the absorption frequency is controlled only with the shape of patterns and the interval of spacers, when the thickness of the spacers is changed by the incident angle of radio waves, the oblique incident property becomes poor.
In JP-A 2003-243876, a theoretical design method is described in which an approximate analysis is conducted on the shape of patterns and the constituents of a radio wave absorber using the FDTD method. However, this method involves various factors to be calculated, and when actually combined with a material used as an architectural interior material with a different complex relative dielectric constant, for example, it is complicated to change the design.
As described in JP-A 06-164184, Japanese Patent No. 3076473 (JP-A 06-244583), Japanese Patent No. 3209456 (JP-A 06-140787), Japanese Patent No. 3209453 (JP-A 06-45782), JP-A 06-252582, JP-A 06-224568, JP-A 09-148782, JP-A 10-224075, JP-A 11-204984, JP-A 11-195890, and JP-A 2003-243876, as a radio wave absorber that can be made thinner and lighter, a patterned radio wave absorber provided with a patterned layer is conceivable, and has been put to practical use as a far-field electromagnetic wave absorber. Herein, the ability to absorb radio waves conflicts with the ability to be thinner and strong. It is difficult to make an absorber thin and to sustain a high strength thereof while keeping a high absorbing ability. In particular, for an electromagnetic wave absorber for absorbing radio waves with a lower frequency such as 1 to 3 GHz, it is necessary that the absorber layer is thick, and thus it is difficult to make the absorber thinner and lighter, and to attain softness and processability in being cut on the spot.
When providing the ability to absorb electromagnetic waves in an architectural interior material such as a ceiling material, a wall material, and a screen, emphasis is put on the workability and the required specification in terms of cost, and thus it is necessary that the electromagnetic wave absorber is thinner, lighter, softer and less expensive to the extent possible. In particular, when a wireless LAN is actually used, metal racks, posts, or air conditioners that have been conventionally present act as electromagnetic wave reflecting layers, and thus there are spots with poor wireless communications. In this case, covering a portion exerting an influence on the wireless communication environment with an electromagnetic wave absorbing member is effective in improving communications, but conventional techniques cannot provide a radio wave absorber that is thin, light, soft, and excellent in the strength and workability while having a high ability to absorb radio waves.